In many digital systems, the interconnection bandwidth between chips is a critical limitation on performance. Historically, inter-chip signaling has performed much more slowly than on-chip processing. As a result, much effort has been focused on increasing bandwidth of signaling between chips since it represents a significant bottleneck for system performance. However, the same problems may develop for signals internal to the chip. As technology continues to scale smaller, the problems with intra-chip signaling will become more pronounced. Without improvements to high speed digital signaling techniques, intra-chip signaling will prove to be a limit to overall system performance.
An example of an ideal digital signal 10 is shown in FIG. 1a. A midpoint 12 is shown that serves to define the change in the value of the data bit. In the lower region 10, the data bit has a value of "0". While in the upper region 14, the data bit has a value of "1". This type of digital scheme with a mid-point 12 is referred to as a single-end signal design. FIG. 1b shows a more realistic view of the waveform of the same digital signal 18. The midpoint 12 as well as the upper 14 and lower 16 regions are the same. However, the signals are subjected to some suppression of the signal's peak value called attenuation. The attenuation is particularly pronounced in the case of a single "1" in a field of "0"s. In some instances, the attenuated signal barely reaches the midpoint 12, which results in a very low probability of detection. The attenuation is primarily caused by skin-effect resistance and dielectric absorption by the transmission line. However, the skin-effect resistance is usually the dominant factor. In any case, the magnitude of the attenuation will increase with the frequency.
With a typical broadband signal, the superposition of an unattenuated low frequency signal component with attenuated high frequency signal components causes intersymbol interference that reduces the maximum frequency at which the system can operate. During this intersymbol interference, or hysteresis, the signal "remembers" its previous state. The problem is not so much the magnitude of the attenuation as it is the interference caused by the frequency dependent nature of the attenuation. The interference comes from noise sources such as receiver offset, receiver sensitivity, crosstalk, reflections of previous data bits, and coupled supply noise.
The effects of such interference are shown in FIGS. 2a and 2b. Both of these figures show a differential digital signal design. The differential signal differs from the single end signal in that it provides complementary high and low signals instead of a single signal. FIG. 2a shows an attenuated differential signal 20. The high signal component 22 and the low signal component 24 intersect to form an eye 26. The amplitude of the eye 28 is obviously dependent on the amount of attenuation of each signal. Only a few decibels (dB) of frequency dependent attenuation can be tolerated by such a signaling system before intersymbol interference overwhelms the signal. FIG. 2b shows a differential signal with deterministic jitter 30. The amount of offset 32 of jitter affects the width of the eye and may possibly eliminate the eye entirely as shown in FIG. 2b. Jitter is caused by fluctuations in the sampling clock, fluctuations in the receiving clock, and delay variations in the signal path. Each of these sources of jitter are primarily the result of power supply modulation and crosstalk induced delay variation.
One solution to the problem of intersymbol interference is equalization of the signal by pre-emphasizing the high-frequency components of the signal before transmission. This will significantly eliminate the interference. The effects of equalization are shown in FIGS. 3a and 3b. FIG. 3a shows an unequalized signal that is similar to that shown in FIG. 2a. As shown previously, the amplitude 28 of the eye 26 of the signal is reduced due to the frequency dependent attenuation. FIG. 3b shows a signal 36 where both the high signal component 22 and the low signal component 24 have been equalized. As can be clearly seen, the amplitude 40 of the eye 38 is increased while the full width of the eye 38 is maintained.